lecture #11 outline - stanford university

October 30, 2002 Stanford University - EE281 Lecture #11 #1

Embedded SystemDesign Laboratory

Lecture #11 Outline

• Motors– DC motors– Stepper motors

• Motor-driving Electronics• RC Servos



DC Motors

• DC motors are distinguished by their ability to operate from direct current (not as evident as it might appear)

• The Parts– Rotor – The rotating center portion. Term may also

refer to the winding that is on the rotor.– Stator – The static (stationary) windings around the

rotor. In many small motors, the stator can be replaced with permanent magnets (not as efficient).

– Commutator – The brush connection to the winding on the rotor.



Simplified DC Motor:

Graphic from application note by MicroMo Electronics at http://www.micromo.com/03application_notes.asp

Stator-inducedconstant B-field

Rotor winding

DC power source



DC Motor Characteristics:

• Speed varies with applied voltage– RPM = (1000*Va)/Ke

• Torque varies with current– Torque = Kt*I

• Polarity determines direction of rotation• Requires an external positional encoder to

close feedback loop.



DC Torque vs. Speed

Graphic from a DC Motor Tutorial Page at http://lancet.mit.edu/motors/motors3.html, © copyright 1999 Center for Innovation in Product Development



Stepper Motors

• Can be thought of as a DC motor without a commutator

• Active windings are on the stator instead of the rotor. Rotor is often a permanent magnet.

• Multiple wires give access to stator coils (typically two coils)

• Requires control electronics for sequencing coils. This replaces the commutator.

• Excellent “Open-Loop” Servo



Two-Phase Stepper Motor

• Power to the two coils must be properly sequenced (phased) to achieve rotor rotation

• Rotation is synchronous to the drive sequence, which means high rotational precision is possible

Graphic from Stepper Motor Application Notes available at: http://www.ericsson.com/microe/apn_ind.html



Bipolar Stepper Motor

• Reversing current flow in the stator windings causes B field to change direction and rotor to move




Bipolar Stepper Motor Drive


H-Bridge



Unipolar Stepper Motor




Unipolar Stepper Motor Drive




Stepper Motor Drive Modes

• Wave-Drive – One phase energized at any instant• Full-Step Drive – Two phases energized at any

instant• Half-Step Drive – One then two phases, every-

other-step• Micro-stepping – Continuously varying current

between phases• Brushless DC Motor – Closed-loop controlled

special case.



Wave-Stepping Sequence

Graphics from: http://eio.com/jasstep.htm

Unipolar motor

Bipolar motor



Full/Half-Stepping Sequence

Graphics from: http://eio.com/jasstep.htm



DC Motor Electronics

• Single Direction– NPN transistor, MOSFET, or power driver IC– ON/OFF control or PWM to control speed, torque, delivered

power– 2N2222 for small motors (I<200mA)– ULN2003/ULN2803 7/8-channel power driver (I<800mA/chip)– TIP120 for larger motors (I<5A)

• Bi-Directional– Need reversible power, an H-Bridge Driver– LMD18200: single H-bridge high-power motor driver. Up to

55V 3A. Accepts enable, brake, and direction inputs– LM18293: dual H-bridge motor driver. I<400mA. May be used

for both stepper motors and DC motors



Stepper Motor Electronics

• Unipolar Drive– Can use 4 to 8 NPN transistors or MOSFETs, or power driver IC

• Must provide sequencing logic/signals to make motor operate• Could do microstepping using PWM• 2N2222 for small motors (I<200mA)• ULN2003/ULN2803 7/8-channel power driver (I<800mA/chip)• TIP120 for larger motors (I<5A)

• Bipolar Drive– Needs reversible power for two coils (Dual H-Bridge Driver)– LM18293: dual H-bridge motor driver. I<400mA. May be used

for both stepper motors and DC motors– UCN5408, MC3479, etc: dual H-bridge driver plus sequencing

logic for stepping the motor. Accepts STEP, DIR inputs.



RC Servos: Overview

• Servos made for Radio-Control applications can be commanded by your AVR!

• RC-Servos are:– Cheap (typically $10, range $5-50)– Strong (typically 42 oz-in or 3.1 kg/cm)– Small (typically 1.6”x0.8”x1.4”)– Have built in control electronics, gearing, mount holes– 3-wire interface (Ground , Power, Control)– Operate on 4-6V– Commanded to a position by pulse width



RC Servos: Commanding

• “Closed-loop” position commanding by pulse width



Motor Web References:

• DC Motors:– http://lancet.mit.edu/motors/index.html– http://www.micromo.com/03application_notes.asp– http://www.instantweb.com/o/oddparts/acsi/motortut.htm (Document still

under development)

• Stepper Motors:– http://www.ericsson.com/microe/apn_ind.html– http://www.st.com/stonline/books/pdf/docs/1679.pdf– http://www.thomsonind.com/airpax/airpax.htm

• RC Servos– http://www.google.com (keywords: RC servo)– http://www.towerhobbies.com

lecture #11 outline - stanford university

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